A vast majority of receptors of the single transmembrane class respond to ligand binding by some form of aggregation. This aggregation can be between identical receptor subunits (as in homodimerization, homotrimerization, etc.) or between different receptor subunits (as is heterodimerization, heterotrimerization, etc). This aggregation appears to be part of the signal for the target cell to respond biologically, in that mutants of the ligand which are unable to interact with the second subunit are still able to bind, but no longer cause dimerization and biological activation of the receptor [P. R. Young, Curr. Opin. Biotech., 3:408-421 (1992)].
For example, there is evidence in the literature that suggests dimerization of the erythropoietin receptor (EpoR) upon ligand binding [S. S. Watowich et al., Molec. Cell Biol., 14:3535-3549 (1992) and S. S. Watowich et al., Proc. Natl. Acad. Sci., USA, 89:2140-2144 (1992)]. Reports about IL-6 have indicated that its second subunit gp130 may dimerize upon IL-6 binding [M. Murkami et al., Science, 260:1808-1810 (1993)]. for some receptors in which homodimerization is induced by ligand binding, monoclonal antibodies (mAbs) were discovered which had agonist properties. These include mAbs to EGF, TNF and growth hormone receptors [A. B. Schrieber et al., J. Biol. Chem., 258:846-853 (1983); L. H. K. Defize et al., EMBO J., 5:1187-1192 (1986); H. Engelmann et al., J. Biol. Chem., 265:14497-14505 (1990); and G. Fuh et al., Science, 256:1677-1680 (1992)]. In each case, these mAbs, by virtue of their two antigen recognition sites, were able to bring together two receptors and thus activate them. Fab fragments made from these mAbs were inactive. In some cases, the apparent affinity of the antibody for receptor was comparable to that of the ligand, e.g., growth hormone [Fuh et al., cited above].
It has also been discovered that antibodies to IL-3 receptor have agonist properties [Suguwara et al., J. Immunol., 140:526-530 (1988)]. Previous literature has described the production of anti-erythropoietin receptor antibodies [A. D'Andrea et al., Blood, 82:46-52 (1993); A. D'Andrea et al., Blood, 84:1982-1991 (1994) and M-G Yet et al., Blood, 82: 1713-1719 (1993). See also, PCT Application WO96/03438 published Feb. 8, 1996. While the Yet et al., reference suggests the occurrence of possible EPO-like activity in one mAb, the mAb is not characterized. Neither Yet et al., nor the other literature provides any reproducible manner of generating agonist mAbs.
There remains a need in the art for the development of additional mAbs which have an affinity for receptors comparable to that of the ligand, and which can act as agonists of the receptor.